The Transverse Process of the Atlas as an Extension of Skull Base Anatomy and Surgical Focal Point

 

Background The transverse process of the atlas (TP-C1) has been identified as an important landmark in several surgical approaches to the posterolateral skull base, but its anatomical relationships have not been systematically described. Its key role in this region stems from its shared developmental anatomy with the skull base, and from its proximity to major neurovascular structures. In this study, we develop a comprehensive scheme for understanding the complex region surrounding the transverse process.

Methods Six cadaveric heads (12 sides) were dissected to expose the transverse process and its anatomical relationships from the lateral and posterolateral perspectives. We examined the related muscles, bony landmarks, neurovascular elements, and previously described surgical triangles. Measurements of the distance from the transverse process to several important structures were made. Our findings were analyzed in the context of the existing anatomical, surgical, and developmental literature.

Results From both a surgical and an artistic perspective, the TP-C1 lies at an anatomical focal point toward which many of the posterolateral skull base structures converge. Based on this observation, we devised a framework for understanding this region that partitions it into angles defined by anatomical structures, all centered at the transverse process. Together, these angles can be visualized as pieces of a pie. Muscles, such as the superior and inferior oblique, as well as imaginary lines such as the “transverse-asterion” line are used to define these angles. The transverse-asterion line also roughly divides the posterolateral region into four major surgical quadrants: the transmastoid presigmoid superolaterally, the suboccipital retrosigmoid superoposteriorly, the jugular foramen inferolaterally, and the far lateral approach inferoposteriorly. Knowledge of the typical distance of the transverse process from other important structures such the spinal accessory nerve, facial nerve, and vertebral artery can also be helpful when exposing this area.

Conclusion Even though the transverse process is an extremely useful landmark for posterior and lateral skull base approaches, its anatomical relationships are sometimes overlooked. From a surgical perspective, it is clear that the transverse process should be treated as a crucial part of skull base anatomy. Similar to the triangles of the cavernous sinus, the scheme we have described should facilitate such an understanding.